1. Technical Field
The invention relates to circuits for coupling electronic signals across a signal path. More particularly, the invention relates to a ground return for use where high speed digital signals are capacitively coupled across a DC-isolated interface.
2. Description of the Prior Art
Various data transfer techniques have evolved as the need to move information, in terms of both speed and quantity, has increased. One promising technique is provided by the Firewire (IEEE 1394) specification. Proposed actual data rates (i.e. independent of any encoding scheme) for Firewire are in multiples of .about.100 Mbit/s.
A central component of the Firewire specification is the link layer--physical layer interface, which is typically implemented in two discrete integrated circuits ("ICs"). Data are transmitted between these two ICs on a data bus. The data bus between the physical layer IC and the link layer IC is clocked at the same rate for all supported speeds. The data rate is increased by widening the bus. Thus, the physical layer uses two bits for 100 Mb/s data transfers, four bits for 200 Mb/s data transfers, and eight bits for 400 Mb/s data transfers.
A two bit wide CNTL bus carries control information. An LREQ pin is used by the link layer to request access to the serial bus and to read or write physical layer registers.
There are four basic operations that may occur in the link layer--physical layer interface, i.e. request, status, transmit, and receive. All of these but request are initiated by the physical layer. The link layer uses the request operation to read or write an internal physical layer register or to ask the physical layer to initiate a transmit action. The physical layer initiates a receive action when a packet is received from the serial bus.
It is necessary to isolate the link layer IC and the physical layer IC electrically when a difference in potential exists between their respective grounds (GND.sub.L and GND.sub.P). The Firewire specification requires galvanic isolation between a cable signal ground and a device's earth ground. Thus, a particular device connected to the link layer, e.g. a CPU or disk drive, may present the need for isolation. Electrical isolation may also be implemented when the link layer is disconnected and the physical layer is powered via a cable.
FIGS. 1a and 1b are block schematic diagrams electrical isolation in a typical Firewire device. Such isolation in Firewire is achieved by an isolation barrier 12 between the physical layer (PHY) interface circuit 14 and the link layer (LINK) circuit 16. There are 6-12 high speed digital signals crossing this PHY-LINK boundary, including a 50-MHz clock and 25-MHz control and data signals. The IEEE 1394 specification allows both magnetically coupled (FIG. 1a) and capacitively coupled (FIG. 1b) isolation circuits for these signals.
Both the IEEE 1394 specification (see J.6 Isolation Barrier, pp. 347, IEEE Std. 1394-1995) and manufacturers of PHY/LINK devices (for example, see Link--Phy Electrical Isolation, IBM21S850PFC IEEE 1394-1995 400 Mb/s PHYsical Layer Transceiver, Application Notes, Version 0.9, pp. 29 (May 9, 1997)) provide for the use of capacitive isolation. However, none of the presently available approaches address the issue of grounding. Thus, while typical implementations of Firewire may include one or two ground return capacitors, such capacitors are not necessarily adjacent to the signal lines.
Accordingly, there has been some difficulty in implementing the PHY/LINK isolation layer. For example, some of the problems encountered when implementing Firewire include self inductance of each signal current path and mutual inductance, i.e. coupling, between signal paths; self-induced voltage drops in each ground return path, resulting in crosstalk between the signals; ground bounce, resulting in slow settling times; and common-mode voltage induced between the grounding systems, resulting in radiated emissions from cables that are attached to isolated grounds.
It would be advantageous to provide a technique for such communications protocols as provided, for example by Firewire, that produces improved signal integrity, reduced generation of electromagnetic emissions, and reduced susceptibility to electromagnetic interference.